Dear Vintage - Opel friends,
I'm pleased to be able to offer you the following article:
Seal thermostat cap on thermostat housing, Opel cih
If the cap of your 4- or 6-cylinder cih engine is attached with exactly two M8 screws, then this paper seal belongs between the cap and the thermostat housing. This is an wonderful rebulit part from edelschmiede :-).
By the way, directly underneath this you will find the thermostat and under that there is another paper seal. This is often happily forgotten about. You can find it naturally here too...